Natural killer (NK) cells provide host defense not only against tumors but also viral and intracellular infections including cytomegalovirus, T. gondii, and L. monocytogenes that are important pathogens in neonatal infections. Since NK cells are present in fetal lymphoid tissues before maturation of T or B lymphocytes, NK cells may be important immune effector cells in fetal and neonatal immunity. NK cells can lyse infected and tumor cells and produce cytokines but their specific activation molecules are unknown. Our long term objective is to determine the functional development of NK cells by determining the cell surface molecules through which adult NK cells are activated, the effector function of these stimulated cells, then functions of developing NK cells. The NK1.1 molecule is the most specific marker on murine NK cells. Previous studies from our laboratory demonstrated that anti-NK1.l monoclonal antibody (mAb) PK136 specifically stimulated both freshly isolated and IL-2 activated NK cells to lyse targets that were not spontaneously lysed by these effector cells. This occurred in a manner similar to T cell lysis of an irrelevant target by anti-CD3 or anti-T cell receptor mAbs. Our preliminary data now shows that immobilized mAb PK136 stimulates NK cells to proliferate in the presence of IL-2. This represents specific stimulation because isotype-matched control mAb did not stimulate. Therefore, we have developed a novel system through which NK cells can be specifically activated. Thus, we propose the following Specific Aims: 1) Examine specific stimulation of NK cells. We will characterize the anti-NK1.1 stimulation in more detail. We will define the modulatory role of other cyokines, particularly IL-12, TNF-alpha, and IL-10, on anti-NK1.1 stimulation. We will examine the response of NK cells to mAbs that activate through other cell surface molecules, including CD69, Ly-6, and Thy-1. 2) Examine the effector functions of the stimulated NK cells. We will examine the ability of anti-NK1.1- stimulated cells to lyse targets and mediate antibody mediated cellular cytotoxicity in 51Cr release assays. We will determine the cytokine production profile of the stimulated cells. 3) Investigate the functional development of NK cells. Having characterized specific activation of adult NK cells, we will then examine NK cells at varying stages of fetal and neonatal development in terms of ability to be stimulated, cytolytic function , and cytokine production. These studies should lead to the precise identification of the functional role of the NK cell in the immune response of the developing fetus and neonate.